Granulocyte colony-stimulating factor (G-CSF) critically affects all stages of granulopoiesis by activating a signaling cascade initiated by dimerization of its receptor (G-CSFR). Five human G-CSFR isoforms have been identified (classes I-V). A quantitative polymerase chain reaction (Q-PCR) technique was used to examine the expression of these five isoforms in normal and leukemic myeloid cells. We demonstrated that neutrophils expressed predominantly the class I isoform and low levels of class IV isoform (IV/I = 0.037 +/- 0.005). No expression of the class II, class III, or class V isoform was detected. In contrast, all AML cell lines and acute myelogenous leukemia (AML) patient samples expressed increased relative amounts of the class IV isoform (IV/I = 0.047-0.350). When compared to normal immature myeloid cells, as represented by the CD34+ fraction of adult bone marrow (ABM) cells, three of eight AML cell lines and three of six AML patient samples expressed significantly increased levels of the class IV isoform relative to class I. This suggests that the increase in the relative expression of the class IV isoform seen in a considerable portion of AML cell samples is related to their leukemic phenotype. Given the inability of the class IV G-CSFR to drive myeloid maturation, the relative increase in class IV expression in AML cells may contribute to their aberrant response to G-CSF.